wito

It's also quite ready to transform this file to JSON and send it to translators through any service that supports his format.

If you are using TypeScript it's quite easy to create a system where the type system will enforce the existence of all translations. I think it should be possible to create a similar solution for other languages as well.

For example:

const enTranslations = { MENU: '' };

const plTranslations: typeof enTranslations = { MENU: '' } as const;

const t = (key: keyof typeof enTranslations) => get language() == 'pl' ? plTranslations[key] : enTranslations[key];

Missing keys will fail compilation. If you want to skip check you can always use //@ts-ignore

Additionally the type system will enforce only valid translation keys so you won't be able to make a typo it forget to add English translation.